1. Field of the Invention
The present invention relates to a network interconnection system for connecting various kinds of networks.
2. Description of the Related Art
There have been various communication networks based on their own individual technical backgrounds, such as a network that connects general telephone sets, a network that connects portable telephones or computers. In these networks, communications are being carried out based on their own standards and their own signal formats (protocols). For example, voice signals are transmitted in the network that connects general telephones. Further, in a LAN (local area network), packet signals having various formats may be transferred. It is not possible to mutually connect these networks and to freely transmit information across these different kinds of networks.
In recent years, demand for connection of these networks has been growing from the viewpoint of the expansion of network infrastructure, cost reduction, and effective utilization of past assets. In order to meet this request, there have been proposed systems for converting signals that are transmitted across these networks.
FIG. 1A shows a conventional network interconnection system. In FIG. 1A, network interfaces (I/Fs) 101–103 are connected to the following network, respectively: X.25 network 104 on which X.25 packets are transferred; ATM network 105 on which ATM (asynchronous transfer mode) packets (or cells) are transferred; and Ethernet network 106 on which Ethernet packets are transferred. Further, these network interfaces 101–103 are also connected to a hub HUB 107. Ethernet represents a typical product name of a LAN that has been developed mainly by Xerox.
FIG. 1B shows an internal structure of the network interface 101. The other network interfaces 102 and 103 also have basically the same structure as that of the network interface 101, and therefore, their explanation will be omitted here. The network interface 101 has a packet converter 111 that is connected to the HUB 107 as shown in FIG. 1A, and a network interface circuit 112 that is connected to the other end of this packet converter 111 and is also connected to the X.25 network 104.
In this conventional network interconnection system, an X.25 packet that has been received from the X.25 network 104, for example, is input to the packet converter 111 via the network interface circuit 112. In the packet converter 111, the X.25 packet is converted into a specific packet (hereinafter to be referred to as a common packet) that is common to these networks. In the case of transfer of the data conveyed in the common packet to the ATM network 105, the common packet obtained by conversion from the X.25 packet is transmitted to the network interface 102 via the HUB 107. The network interface 102 has a packet converter similar to the packet converter 111 as shown in FIG. 1B. However, the packet converter of the network interface 102 is different from that of the network interface 101 in that a conversion is performed between a common packet and an ATM cell. ATM cells obtained by conversion in the packet converter of the network interface 102 are transmitted to the ATM network 105 via the network interface circuit within the network interface 102. The data conversion between the X.25 network 104 and the Ethernet network 106 and the data conversion between the ATM network 105 and the Ethernet network 106 are carried out in a similar manner.
However, in such a conventional system configuration as shown in FIGS. 1A and 1B, each time a new standard or a new signal format is employed, an interface and a processor supporting these new one become necessary. Therefore, such a conventional system has a problem of lack in extensibility as a hardware function. To cope with this situation, there have been proposed methods of controlling a compound switching system that can accommodate various terminals of different signal formats. According to a proposal disclosed in Japanese Patent Application Unexamined Publication No. 58-151748, data transmitted from a subscriber terminal is converted into a uniquely defined packet that is used in a packet network within the switching system.
FIG. 1C shows a format of a uniquely defined packet that is used in the packet network within the switching system. A packet 121 has flags 122 and 123 at the front edge and the rear edge thereof, that are similar to in the HDLC (high-level data link control) procedure. The packet 121 also has an source and destination address field (ADR) 124, an information field (DATA) 125, and an error correcting code field (CHK) 126. The source and destination address field (ADR) 124 includes a source address and a destination address. The information field 125 includes the contents of a transmitted signal. Based on one common format for signals handled within the switching system as shown in FIG. 1C, it is possible to simplify the signal processing when data having various signal formats have been taken into the switching system, without the need of preparing different hardware for each signal format. In other words, it is possible to guarantee the extensibility of the hardware function.
According to Japanese Patent Application Unexamined Publication No. H9-233122, there has been disclosed an inter-LAN connection router for connecting LANs via an ISDN (Integrated Services Digital Network) network. Based on the conventional technique, protocol-type packets having a specific signal format set in advance by a user are transferred by a D-channel packet switching and other packets are transferred by a B-channel packet switching, resulting in efficient data communications.
As described above, there have conventionally been techniques for converting data into a specific-format data packet that can be handled in common within the switching system. Contrarily, for transmitting these signals to networks outside tho switching system, the signals are converted again into signal formats corresponding to the outside networks, and then the converted signals are transmitted.
In recent years, data communications between computers have increased rapidly with the widespread of LANs and the Internet. Under this situation, there has been a possibility that a major portion of various information taken into the switching system and transmitted to the networks is data to be used by other computers via the LANs and the Internet. As a representative type of this data, there is data based on a MAC (media access control) frame used in a MAC layer. The MAC layer is a lower sub-layer of the data link layer, which is composed of an LLC (logic link control) layer as a upper sub-layer and the MAC layer as a lower sub-layer.
According to the above-described conventional techniques, however, an input signal is converted into a signal format that is used in common within the switching system. This signal format is not determined taking into consideration a network at the output side (transmission side) of the switching system. Accordingly, it is necessary to make substantial changes in signal format at both times of taking a signal into the switching system and transmitting a signal to outside. This results in a complicated signal conversion processing.